1. Field of the Invention
The invention relates to a pressure control device for an inkjet pen; in particular, a pressure control device that can enhance assembly yield and reduce assembly time.
2. Description of the Related Art
Conventional ink-jet printing generally relies on the controlled delivery of ink droplets from a reservoir of an inkjet pen to a print medium. Among the printing methods for delivering ink drops from the reservoir to the print head, drop-on-demand printing is known as a commonly used method. Drop-on-demand typically uses thermal bubble or piezoelectric pressure wave mechanisms. A thermal bubble type print head includes a thin film resistor that is heated to cause sudden vaporization of a small portion of ink. The vapid expansion of the ink vapor forces a small drop of ink through a print head nozzle. Although drop-on-demand printing is ideal for sending ink drops from a reservoir to the print head, some mechanism must be included to prevent ink leaking out from the print head when the print head is inactive. Such a mechanism usually can build a slight back pressure at the print head to prevent ink leakage from the pen whenever the print head is inactive. Herein, the term xe2x80x9cback pressurexe2x80x9d represents the partial vacuum within the reservoir. Back pressure is defined in the positive sense so that an increase in back pressure means the degree of partial vacuum has increased.
When the back pressure is established at all times inside the reservoir, ink is prevented from permeating through the print head. However, the back pressure can not be so high that the print head is unable to overcome the back pressure to eject ink drops. Furthermore, as ambient air pressure decreases, a correspondingly greater amount of back pressure is needed to keep ink from leaking. Accordingly, the back pressure within the inkjet pen has to be regulated whenever ambient pressure drops. Also the pressure within the pen is subjected to what may be termed xe2x80x9coperational effectsxe2x80x9d, as the depletion of ink from the reservoir increases the back pressure of the reservoir. Without regulation of this back pressure increase, the inkjet pen will fail soon because the back pressure is too high for the print head to overcome it and eject ink drops.
Conventionally, the back pressure within the reservoir is controlled by mechanism referred to as accumulators. In general, an accumulator includes an elastomeric bag capable of moving between a minimum volume position and a maximum volume position in response to changes in the back pressure within the reservoir. For example, as ambient pressure drops so that back pressure within the reservoir decreases simultaneously, the accumulator will move to increase the volume of the reservoir to thereby increase the back pressure to a level that prevent ink leakage. Another example is depletion occurring during operation of the pen. In such a case, accumulators will move to decrease the volume of the reservoir to reduce the back pressure to a level within operating range, thereby permitting the print head to continue ejecting ink.
However, although accumulators such as elastomeric bags can automatically adjust the volume of the reservoir to keep the back pressure within the operating range, the extent to which elastomeric bags are capable of expanding is quite limited. Consequently, when ink level gradually drops from the print head, the bag may reach its maximum extent and therefore incapable of any further adjustment of the volume of the reservoir. Hence, the back pressure within the reservoir may increase such that ink droplets are prevented from leaving the print head.
To resolve the aforementioned problems, some inkjet pens employ a device called a xe2x80x9cbubble generatorxe2x80x9d. The bubble generator has an orifice through which ambient air can enter the reservoir. The dimension of the orifice is such that ink is trapped within the orifice to seal off the reservoir by capillary effect. When ambient air pressure is high enough to overcome the liquid seal, air can bubble into the reservoir. Therefore, the back pressure within the reservoir can decrease and capillary effect will take over and re-establish the liquid seal again to prevent entrance of more air bubbles.
In general, bubble generators of inkjet pens must satisfy a few conditions. Firstly, the bubble generator must be able to control back pressure precisely. Secondly, the range of fluctuation of the back pressure within the reservoir must be as small as possible. In other words, as air bubbles enter the reservoir leading to a drop in back pressure, the bubble generator must be able to stop the entrance of bubbles soon enough that a suitable back pressure remains inside. Thirdly, the bubble generator must have self-wetting capability. The liquid seal must be able to prevent the entrance of bubbles even when most of the ink within the reservoir is used up, or alternately when the inkjet pen is tilted so much that the bubble generator is no longer immersed below the ink.
Referring to FIG. 1 and FIG. 2, a conventional bubble generator 118 according to U.S. Pat. No. 5,526,030 is shown. The bubble generator 118 installed within the reservoir 102 has an orifice 122 and a sphere 124. FIG. 2 is a top view showing the surrounding structure of the bubble generator 118. As shown in FIG. 2, the internal side-walls of the orifice 122 contain equidistantly spaced protruding ribs 126, 128 for centering the sphere 124. The circular gap 120 between the sphere 124 and the orifice 122 is located where ambient bubbles are produced. Normally, a bubble generator 118 as above is able to meet the demands required for printing with an inkjet pen. In general, the entrance of bubbles into the inkjet pen is determined by surface tension of the ink itself, static pressure of the ink column and the gap 120 between the sphere 124 and the orifice 122, as shown in FIG. 3. Usually, the greater the surface tension of the ink or smaller the gap between the sphere 124 and the orifice 122, the higher will be the back pressure required within the reservoir 102 before air bubbles will start to enter. In addition, the static pressure of the ink column within the reservoir 102 can affect the value of back pressure required before air bubbles begin to enter the reservoir. Therefore, as ink level gradually drops, static pressure of the ink column will decrease leading to the entrance of air bubbles at a smaller back pressure. In summary, major drawbacks of the aforementioned pressure control technique includes:
1. The value of back pressure within the reservoir before the bubble generator starts to function is related to surface tension of the ink used. Since various inks may have different surface tension, the minimum back pressure under which air bubbles can enter the reservoir may be different for each type of ink. Consequently, the gap between the sphere and the orifice must be designed for various inks.
2. The value of back pressure within the reservoir before bubble generator starts to function is also related to the static pressure generated by the column of ink. As ink level within the reservoir drops gradually, static pressure acting on the bubble generator will drop making it easier for air bubbles to enter the reservoir. Often this will lead to a lowering of back pressure within the reservoir, and the adjustable range of the accumulator will be reduced.
3. The gap between the sphere and the orifice has to be precisely engineered to permit the entrance of air bubbles at the correct back pressure within the reservoir. This will increase difficulties in fabricating the reservoir of an ink-jet pen.
FIG. 4 shows another conventional pressure control device 410 for an inkjet pen according to U.S. Pat. No. 6,213,598. During the assembly, a flat spring 330 is welded to the bottom of an inkjet pen 400 so that the flat spring 330 presses a sphere 320 of a bubble generator. Since the flat spring 330 is located at the bottom of the inkjet pen, it is difficult to dispose the flat spring 330 at a predetermined position during the assembly. In addition, an expandable bag 416 is in contact with a pressure plate 412, and the pressure plate 412 is supported by a spring 414. The arrangement between the pressure plate 412, the spring 414, and the bag 416 is unreliable, and the assembly between them is laborious.
In light of the foregoing, there is a need to provide a better pressure control device within a reservoir.
In order to address the disadvantages of the aforementioned pressure control device for inkjet pen, the invention provides a pressure control device that can enhance assembly yield and reduce assembly time.
Another purpose of this invention is to provide an inkjet pen with a stable pressure control device.
Accordingly, the invention provides a pressure control device for a reservoir having a first opening and maintaining back pressure established therein. The pressure control device comprises a bag, a pressure plate, an isolation member, a bias member, and an elastic member. The bag, disposed inside the reservoir, communicates with outside the reservoir so as to expand inside the reservoir. The pressure plate is disposed inside the reservoir and adjacent to the bag so as to move inside the reservoir. The isolation member, disposed inside the reservoir in a moveable manner, seals the first opening. The bias member is disposed inside the reservoir and adjacent to the pressure plate, the isolation member, and the reservoir respectively so as to move inside the reservoir. The bias member adjusts the isolation member to seal the first opening based on the movement of the pressure plate. The elastic member, disposed inside the reservoir and adjacent to the pressure plate and the bias member respectively, restrains the expansion of the bag. Thus, the bag expands to move the pressure plate when the back pressure inside the reservoir changes, then the pressure plate moves the bias member so that the isolation member separates from the first opening.
In a preferred embodiment, the reservoir is provided with a second opening, and the bag is provided with a third opening communicating with the second opening.
In another preferred embodiment, the isolation member is a sphere.
In another preferred embodiment, the bias member is provided with an extension plate, adjacent to the reservoir, for the convenience of the assembly of the bias member.
Furthermore, the extension plate is provided with at least one support for fixing the elastic member, and the extension plate is fixed on the reservoir.
In another preferred embodiment, the bias member is a flat spring.
In another preferred embodiment, the elastic member is a spring.
In another preferred embodiment, the outside of the reservoir refers to the atmosphere.
In another preferred embodiment, the bias member and the elastic member are made of stainless steel.
In another preferred embodiment, the bias member and the elastic member are integrally formed.
Furthermore, the reservoir is provided with at least one post, and the integrally formed bias and elastic member is provided with at least one first through hole. Thus, the integrally formed bias and elastic member is fixedly disposed inside the reservoir by inserting the post through the first through hole.
Furthermore, the post is fixed inside the first through hole by welding.
Furthermore, the reservoir is provided with a rib for fixing the integrally formed bias and elastic member.
Furthermore, the integrally formed bias and elastic member is provided with at least one second through hole for adjusting the elastic coefficient of the integrally formed bias and elastic member.
In another embodiment, the invention provides an inkjet pen. The inkjet pen comprises a reservoir, a bag, a pressure plate, an isolation member, a bias member, and an elastic member The reservoir has a first opening and maintains a back pressure established therein. The bag, disposed inside the reservoir, communicates with outside the reservoir so as to expand inside the reservoir. The pressure plate is disposed inside the reservoir and adjacent to the bag so as to move inside the reservoir. The isolation member, disposed inside the reservoir in a moveable manner, seals the first opening. The bias member is disposed inside the reservoir and adjacent to the pressure plate, the isolation member, and the reservoir respectively so as to move inside the reservoir. The bias member adjusts the isolation member to seal the first opening based on the movement of the pressure plate. The elastic member, disposed inside the reservoir and adjacent to the pressure plate and the bias member respectively, restrains the expansion of the bag. Thus, the bag expands to move the pressure plate when the back pressure inside the reservoir changes, then the pressure plate moves the bias member so that the isolation member separates from the first opening.